TABElectronicsGuidetoUnderstandingElectricityandElectronics-1

332 Chapter Twelve

(Fig. 6-11), and the noninverting input would connect to the anode of
D1 (the voltage reference diodes in Fig. 6-11). The output of the op amp
connects to the base of Q1 (the series-pass transistor, Fig. 6-11), and the
op amp power supply connections are made to the positive and circuit
common points as illustrated.

The circuit, shown in Fig. 12-2, is included as a basic reference for all
of your future power supply needs. To use this design, start by design-
ing a simple zener-regulated power supply (the transformer, bridge recti-
fier, filter capacitor, R1, and D1). Be sure that the zener voltage of D1 is
somewhat less than the anticipated voltage drop across R3. The equation
for calculating the value of R2 and R3 is provided in the illustration. Of
course, Q1 is chosen according to the current and power dissipation
requirements of the proposed loads to be applied to the circuit.

The best method of improving the lab power supply is illustrated in
Fig. 12-3. The full regulator circuit design has been included so that the
circuit modifications might be more easily located. These alterations
should be self-explanatory by comparing Fig. 6-11 with Fig. 12-3. Note
that you will need to mount two additional binding posts for the posi-
tive and negative regulated outputs. Keep the original two posts intact,
connected to the unregulated current limit circuit, for testing audio
power amplifiers (and other projects requiring higher-voltage, dual-
polarity supplies). Also note that P1 and P2 (Fig. 6-11) must be replaced
with 5-Kohm potentiometers.

Building a Quad Power Supply

Figure 12-4 illustrates a quad-output lab power supply. This is an
extremely versatile lab power supply. It provides the most frequently
used voltages in dual polarity. Each output is current-limited at 1.5 amps,
and all of the regulator ICs are internally protected from overvoltage
and overtemperature conditions. The regulator ICs are of the fixed-voltage
type, meaning that they are not voltage-adjustable, but they provide
extremely good voltage regulation. All of the power supply components
have been chosen so that all four outputs can be loaded to maximum
capacity simultaneously.

All four regulator ICs will require some form of heatsinking. Mounting
the ICs to a metal enclosure will probably be sufficient, but the combina-
tion of the chassis and the “sinks” is recommended. After constructing the
power supply, load down all four outputs, and let it run for a while. If the

Integrated Circuits 333

Figure 12-3
Improvement of origi-
nal lab power supply
illustrated in Fig. 6-11.

Figure 12-4 2600 1 NTE966 3 ϩ 12V
A quad-output lab ␮F 2
power supply.
T1 ϭ 25.2V C.T. @ 6A 2600 1 ␮F 1 ␮F 100 ␮F
White ␮F

1 ␮F 1 1 ␮F 100 ␮F
2 NTE967 3
120 V Ϫ 12V
ac
Ϫϩ

SW1 Br1 1 NTE960 3 ϩ 5V
8A 2 100 ␮F
F1 50 PIV
2A 1 ␮F 1 ␮F 100 ␮F
1000 Ϫ 5V
␮F 1 ␮F
1 1 ␮F
1000
␮F 2 NTE961 3

Black

Green 120 V
ac Plug

334 Chapter Twelve

enclosure (or the ICs) start to become too hot, you will need to include
some additional heatsinking between the enclosure and IC mounting tabs.

Circuit Potpourri

Noise Hangs in the Balance

Most high-quality microphones, those designed for professional enter-
tainment use, are low-impedance devices and have balanced XLR-type
connector outputs. Although these “mikes” can be impedance matched
for a high-impedance input with a single transistor stage, this will defeat
the whole purpose of having a balanced line.

The circuit illustrated in Fig. 12-5 will match the impedance correctly,
and utilize the high common-mode rejection characteristic of opera-
tional amplifiers to remove the unwanted “hum” picked up by long
microphone wires.

Hum Reducer

Furthermore, on the subject of hum (stray 60-hertz noise, inductively
coupled to an audio circuit), the circuit illustrated in Fig. 12-6 can do a
very effective job of removing almost all hum content, even after it has
already been mixed with other audio material. However, it will also
reduce some low-bass frequencies (from about 40 to 100 hertz).

Figure 12-6 shows a “twin-tee” notch filter. The component values cho-
sen will cause the output to drastically attenuate 60-hertz frequencies,

Figure 12-5 1 k⍀ 10 k⍀ Out
A balanced input cir-
cuit for low-imped- 1k Vϩ
ance microphones. 10 k⍀
2Ϫ 8 1
XLR 3ϩ 4 4.7 k⍀
Jack 10 k⍀ VϪ

10 k⍀

Op amp ϭ NTE778A

Integrated Circuits 335

Figure 12-6 R1 ϭ R2 R3 ϭ R1 1
A 60-hertz notch C1 ϭ C2 2 FN ϭ 2␲R1C1
filter. Output
C3 ϭ2(C1)

R1 R2 2 ϩV 1
470 k⍀ 470 k⍀ 3 Ϫ8

ICI
ϩ4

C3 ϪV
0.01␮F
Input

R3
220 k⍀

C1 C2 ICI ϭ NTE778A
0.005 ␮F 0.005 ␮F

but allow most other frequencies to pass unaltered. Attenuation at 60

hertz should be on the order of about 12 dB.

Equations are provided in the illustration to design notch filters for

other frequencies, also. Notice the equation for finding the notch fre-

quency (F ). The little symbol with the two legs and a wavy top is called
N

pi. Pi is a mathematical standard, and it is approximately equal to 3.1416.

Let the Band Pass

Figure 12-7 is an example of a bandpass filter. A bandpass filter only pass-
es a narrow band of frequencies, and severely attenuates all others. This
circuit illustrates how a JFET can be used as a voltage-controlled resis-
tor; to “tune” the bandpass frequencies of the filter by means of a con-
trol voltage (about 0 to 2 volts DC). The usable frequency range is from
subaudio to about 3 kHz. This circuit is commonly used to produce
“wah-wah” effects for electric guitars and other instruments.

When several of these circuits are placed in parallel with component
values chosen to provide different ranges of bandpass, the result is a para-
metric filter. If a manual control of the bandpass is desired, Q1 might be
replaced with a 5-Kohm resistor.

High-Versatility Filters

Figures 12-8 and 12-9 are both examples of equal-component, Sallen-Key
active filter circuits. Equal-component filters are easier to build, because